Digital video multiplexing allows multiple encoded video bitstreams to be transmitted on a common digital channel. Concatenating a large amount of data on a safe channel has its benefits, but also presents significant challenges. Due to the inherent characteristics of video content, the bit rate of video data fluctuates. Video content with complex scenes and frequent motion has higher bit rates, whereas videos with simpler scenes have lower bit rates. However, the transmission channel often has a fixed bandwidth or fixed allocated bandwidth for a period of time. Consequently, if the total bit rate exceeds the available channel bandwidth, some of the encoded data is dropped or delayed for later transmission. This results in video quality degradation due to repeated drops and delays of encoded data. Video statistical multiplexing is a technique that takes the advantage of the statistical nature of unrelated video sources to improve video quality. For unrelated video sources, different sources will not always generate high bit rates or low bit rates at the same time. From a mathematical perspective, the aggregated bit rate will statistically have a bell shaped probability distribution curve with a mean bit rate and a variance. When the number of video sources increases, the normalized variance becomes smaller. The aggregated bit rate is thus more “predictable” or “stable” for a larger number of video sources. As a result, the likelihood that the aggregated bit rate will exceed the channel bandwidth decreases. While the video source is addressed herein, video streams are accompanied by associated audio content that is transmitted synchronously. Audio content is often compressed using constant bit rate (CBR) coding or variable bit rate (VBR) coding. The quantity of compressed audio data is usually smaller that that of video data.
For the sake of simplicity, only video data is specifically referred to herein. However, it is understood that video data may include associated audio data. In addition, the terms “bitrate”, “bit rate” and “rate” are often used interchangeably in the art to refer to units of video data per unit of time. Other terms maybe used herein to refer to technical jargon and well known terms of art These terms are used for illustrations purposes.
While statistical video multiplexing takes advantage of the statistical behavior of underlying multiple video sources, the aggregated bit rate may still exceed the available bandwidth. In order to further improve video quality, the statistical video multiplexer can work in conjunction with the video encoders to reduce the aggregated bit rate to fit within the allocated overall bandwidth. The video statistical multiplexer may take a very conservative approach by allocating a relatively low bit rate to each encoder to reduce or duplicate data loss. In such a design, each encoder could use a coarse quantizer or other means to generate a compressed bit stream that fits within the allocated bit rate. As a consequence of the low bit rate, however, degradation in video quality may result. Those skilled in the art may adopt a general bit rate control strategy that allocates as high a bit rate as possible while keeping the aggregated bit rate within the allocated bandwidth.
If the multiplexer is located at a service head-end while the encoders are remotely located at different sites, the statistical video multiplexing is complicated by possible network delays between the multiplexer and encoders. Dynamic delay characteristics result when the multiplexer and video encoders are connected over internet protocol (IP) networks. In an IP network environment, packet delay can often occur during periods of high overall network traffic. To account for this effect, the multiplexer may use a simple approach by always assuming the greatest possible delay for all encoders. However, this simple approach unnecessarily introduces more delay in transmitting the underlying video data generated by the video encoders.
Therefore, there is a need for methods and systems of video multiplexing that minimize delay while maintaining optimal video quality. These methods and systems should take into account the dynamic delay characteristics of IP networking and provide an adaptive solution for the changing network environment. Furthermore, the methods and systems should provide a mechanism for the multiplexer to function properly during the transition from one network delay to another network delay.